Method for increasing the melt viscosity of synthetic linear polyamides



Patented June 19, 1951 METHOD FOR INCREASING 'rnn MELT VISCOSITY F SYNTHETIC LINEAR POLYAMIDES Isaac F. Walker, Hockessin, DeL, assignor to E. I. du Pont de Ncmours & Company, Wilmington. Del., a corporation of Delaware No Drawing. Application June 26, 1946, Serial No. 679,588

This invention relates to polyamide resins having improved properties-and more particularly to a novel method for preparing them.

Synthetic linear condensation polyamides are useful in many applications, such as in the manufacture of fibers, bristles, sheets, tubes and the like. They can be prepared by the methods described in U. S. Patents 2,071,250; 2,071,253; and 2,130,948.

These polyamides, used in the practice of this invention, comprise the reaction product of a linear polymer-forming composition containing amide-forming groups, for example, those obtained by reacting material consisting essentially of bifunctional molecules each containing two reactive groups which are complementary to reactive groups in other molecules and which include complementary amide-forming groups. These polyamides can be obtained by the methods given in the previously mentioned patents and by other methods, for example, by self-polymerization of a monoaminomonocarboxylic acid, by reacting a diamine with a dibasic carboxylic acid in substantially equimolecular amounts, or by reacting a monoaminomonohydric alcohol with a dibasic carboxylic acid in substantially equimolecular amounts, it being understood that these reactants can be replaced'by their equivalent amideforming derivatives. These polycarbonamides have an intrinsic viscosity of at least 0.4,and

have an average number of carbons of at least 2 in the segments of'the chain separating the amide.

groups. The preferred polyamides have a unit length (defined as in Patents 2,071,253 and 2,130,- 948) of at least .7. However, these polyamides possess viscosities which are lower than desirable for use in certain other applications. For example, the melt viscosity of polyhexamethylene adipamide, as prepared by the ordinary methods, is undesirably low for its use in injection molding processes. Also, the transparency of articles cast from some of these polyamides is not as high as desired for certain uses.

I This invention has as an object a new process for modifying linear polyamides. More specifically, this invention has as an object a novel process for increasing the viscosity of such polyamides. It is another object to improve the transparency of articles cast from such polyamides. Still other objects will appear hereinafter. a

These objects are accomplished b methods described herein which comprise vacuum melts blending a synthetic linearpolyamide with a member of the class consisting of polybasic acids higher than dibasic, their anhydrides, esters, amides, and salts.

3 Claims. (01. 260-78) The process of this invention is carried out by first forming a uniform mixture of the synthetic form molten mixture is obtained .(the actual time I required being dependent on the quantity of reactants being mixed) it is stirred rapidly while being subjected to reduced pressure with continued heating to remove the volatile reaction products formed by reaction between the polybasic acid and the polyamide. The absolute pressure used in this vacuum treatment is maintained low enough to efiect rapid removal of volatile materials. Pressures of 100 mm. of mercury are operable; however, it is preferred to use pressures of 0.01 to 10 mm. of mercury. The vacuum treatment is continued at the same temperature until the melt viscosit of the reaction mixture has increased to the desired degree, namely, to an increase of at least 10%. In some cases the vacuum treatment is discontinued as soon as the minimum pressure desired has been obtained,

. more, the exact time depending on the particular while in other cases the treatment is continued at the minimum pressure for a period rangin from several minutes to several hours, e. g., 5 hours or polybasic acid modifier and its concentration, and on the viscosity desired in the final product. An inert gas such as nitrogen is then introduced into the reaction vessel and the reaction mixture is discharged from the reaction vessel and cooled rapidly. If desired, the reaction mixture may be cooled in the vessel and removed later.

The invention is illustrated by the following examples in which the proportions of ingredients are expressed in parts byweight unless otherwise noted.

Example I I Forty-seven and one-half (47.5) parts of polyhexamethylene adipamide' and 0.42 part of'85% phosphoric acid are blended at 270280 C. by stirring in a glass reaction vessel in an'atmosphere of nitrogen. "In about 15 minutes the polymer and phosphbric acid forms a uniform melt of low viscosity. The reaction vessel is evacuated while the reaction mixture is stirred rapidly to minimize frothing. When the pressure reaches about 1 mm. of mercury, stirring is discontinued, nitrogen is admitted to the reaction vessel and the vessel and its contents cooled rapidly by plunging it into cold water. The resultin solid I '3 polymer is found to have a relative viscosity of 52 when measured as an 8.4% solution in 90% formic acid-% water (by weight). The ori inal unmodified polyhexamethyiene adipamide has a relative viscosity of 27, when measured in the same way.

Examples n-xrx is obtained with a vacuum treatment of about 2- minutes.

In addition to producing a desirable increase in vicsosity, as illustrated in the examples, this vacuum melt-blending oi polyamides with polybasic acids, or derivatives thereof, frequently yields products which are more transparent than-the unmodified polyamide. If polyhexamethylene adipamide is melted in a mold and cooled rapidly, e. g. by plunging the mold into ice water, it is found that sections of polymer up to about mils (0.020 inch) thickness are transparent, while thicker sections are translucent or opaque. 0n the other hand some of the modified polyhexamethylene adipamides of the examples have an improved transparency, sections molded in the same way in thicknesses up to 40 mils being Viscosity 0! Parts by Per Cent Product Emmi)! weight Poiybasic Acid or derivative Pan's by of Poly- No, omyweight amide R l e e of Melt tive 857 Phosphoric acid 0.42 0.75 medium 62 85 Phosphoric acid 0. 42 0. 75 gli 84 0.5% Phosphoric acid 3 0.03 do 81 3m mm 0. 5 1. 0 do... 80 Phosphorated octyl alcohol 0. 3 l. 0 -.d0. 124

on 0. l5 0. 5 do 114 Monosodium phosphate monohydrate. l. 75 3. 5 medium Disodium phosphate dodecahydrate 4.3 8. 6 high. Trisodium phos hate 2. l 4.2 v. hig Diammonium p osphate. 2 4 do Ammonium Borate- 1 2 0 medium 44 Tetraphcsphoric acid 0. 5 l. 0 gh Styrene/maleic anhydride interpolyn er 0. 5 i. 0 medium 36 Stearyl half ester of styrene/maleic anhydnde inter- 0. 5 l. 0 do. 53

ymer. Pdigamide from p-amino benzene sullonamide and sty- 2. 5 5 high rene/maleic anhydride mterpolymer.

Tricresyi phosphate.-- 2. 34 4. 7 medium 46 Borotungstic acid l. 0 2. 0 do 45 Mixture of monoaud 5111a 2 4 .do

% Giycerophosphonc acid l 0. 5 dc.

Non low. 27

In-all examples except 3, 4, 5 and 6, the vacuum treatment. was discontinued after a pressure of about 1 mm. of mercury was reached. In Examples 3, 4, and 6, the time of treatment was continued 60 minutes, and in Example 5 for minutes. a

The percent ofpolybasic acid or derivative in the foregoing-table designates the percentage by weight, based on the polyamide, of polybasic acid orderivative actually present in the modifier. The relative viscosity is that of an 8.4% solution of the isolated polymer in a 90% formic acid-- 10% water mixture at 25 C. In the above, and in the other examples the phosphoric acid is orthophosphoric acid unless otherwise designated. 1 The phosphorated octyl alcohol in the table 0 a rived from amino acids and from mixtures of dihas the formula (CaHn) s NasPcozo.

Emample'XXI Fifty parts of poly-G-aminocaproic acid is vacuum melt-blended with 0.05 part of 85% phosphoric acid in the manner described in Example]; A product of very high melt viscosity the practice of this invention, are polytetramethtransparent. This improved transparency is in-' dicated by the results tabulated below:

- Modifier Max. thickness of amide of P ig- I er mo Cent mil Phosphoric Acid- 0.75 Phosphor-sited octyl alcohol 0. 5 Monosodium phosphate 3. 5

Disodium phosphate 8. 6 Trisodium phosphate 4. 2

. Ammonium bcrate 2 Tricresyl phosphate 4. 7 Glycerophosphoric acid 0. 5

Examples of further specific polyamides debasic acids and aliphatic diamines, and useful in yiene sebacamide, polypentamethyiene .adipamide, polypentamethylene sebacamid'e, polyll-aminoundecanoic acid. Interpolyamides can also be used.

Other inorganic or organic acids higher than dibasic, as well as the previously mentioned derivatives thereof, can be used in the present process. Examples of further acids of this kind are:

citric acid, tricarballylic acid, trimesic acid, camacid, pyrophosphoric acid, and arsenic acid.

Phosphoric acid and its anhydrides, esters, amides, and salts are particularly useful in the process of this invention.

The esters useful in the practice of this invention are esters of the above-defined acids with monoor polyhydric alcohols or phenols. Since the process involves heating the polyamide and polybasic acid derivative at relatively high temperatures, it is preferable to employ esters which are relatively non-volatile. For this reason, esters of long chain alcohols or of phenols are preferred. Further examples of esters which'are operable include: butyl esters of styrene/maleic anhydride interpolymers, butyl polysilicate, triphenyl phosphate, and the like.

The amides useful in the practice of this invention are N-substituted and unsubstituted amides of the above-defined acids. Other specific amides which are operable include: N-butyl phosphonamide, citramide, and the anilide of styrene/ maleic anhydride interpolymer.

Salts of the above-defined acids with ammonia, alkali metals, and alkaline earth metals can be employed in the process: of this invention. The ammonia and alkali metal salts are particularly useful. Further specific examples of salts which are operable include potassium phosphate, magnesium pyrophosphate, calcium hypophosphite, magnesium citrate, and the like.

, The anhydrides of any of the above-defined acids can also be used in the process of this invention. In addition to silica aerogel, other examples include boron trioxide, phosphorus pentoxide, tricarballylic anhydride acid, and the like.

The proportions of polybasic acid, or derivative thereof, used inthe process of this invention can be varied over a considerable range. .In general, proportions ranging from 0.01% to of the weight of the synthetic linear polyamide are satisfactory. Preferably, amounts ranging from 0.02% to 5%. of the polyamide are employed. The particular concentration of modifier selected depends on several. factors, such as its type and the particular degree of viscosity increase desired ployed, a larger proportion ofmodifier within the I above stated range'is used, and the treatment is continued for a longer time.

The products of this invention are particularly useful in applications where polyamides having high viscosity are desired, e. g., in injection molding processes and where cast articles of im-.

proved transparency are desired. They are also useful for the manufacture of fibers, bristles, sheets, tubes, etc.

As many apparently widely different embodi-.

ments of this invention may be made without J departing from the spirit and scope thereof, it is in the polyamide being treated. Thus, to obtain a given increase in viscosity with a given polyamide; the'more effective polybasic acid modifiers, e. g., phosphoric acid and its derivatives are used in the. lower concentrations of this range. However, if a greater increase in viscosity is desired, or less effective modifiers are emto be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

I claim: 1. A process for increasing the melt viscosity of synthetic linear polyamides which comprises" heating in an inert atmosphere a mixture of a preformed polyamide having an intrinsic viscosity of at least 0.4 with from 0.01% to 10%, by

weight thereof, of phosphoric acid, said heating being at a pressure below mm. of mercury at which volatile reaction products are removed, and being at a temperature above the melting point of the polyamide but below the temperature at which substantial degradation of the polyamide takes place, said polyamide having recurring hydrogen-bearing amide groups as an integral part of the main polymer chain, and

.having an average number of carbon atoms of at least two in the segments of the chain separating the amide groups.

2. The process set forth in claim 1 in which said phosphoric acid is present in the proportion of 0.02% to 5% by weight of said polyamide. 3. The process set forth in claim 2 in which said polyamide is polyhexamethylene adipamide.

ISAAC F. WALKER.

REFERENCES CITED The following references are of recordin the file of this patent:

UNITED STATES PATENTS A Certificate 0!. Correction I Patent No. 2,557,808 June 1951 ISAAC F. .WALKER It is hereby certified that error appears in the printed specification of the ,above numbered patent requiring correction as follows:

Column 3, line 65, for 200 C. read 280 0.;

and that the said Letters Patent should be read as correetedabove, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 21st day of August, A. D 1951.

THOMAS MURPHY,

Assistant Oomz'saimer of Paten'ta. 

1. A PROCESS FOR INCREASING THE MELT VISCOSITY OF SYNTHETIC LINEAR POLYAMIDES WHICH COMPRISES HEATING IN AN INERT ATMOSPHERE A MIXTURE OF A PREFORMED POLYAMIDE HAVING AN INTRINSIC VISCOSITY OF AT LEAST 0.4 WITH FROM 0.01% TO 10%, BY WEIGHT THEREOF, OF PHOSPHORIC ACID, SAID HEATING BEING AT A PRESSURE BELOW 100 MM. OF MERCURY AT WHICH VOLATILE REACTION PRODUCTS ARE REMOVED AND BEING AT A TEMPERATURE ABOVE THE MELTING POINT OF THE POLYAMIDE BUT BELOW THE TEMPERATURE AT WHICH SUBSTANTIAL DEGRADATION OF THE POLYAMIDE TAKES PLACE. SAID POLYAMIDE HAVING RECURRING HYDROGEN-BEARING AMIDE GROUPS AS AN INTEGRAL PART OF THE MAIN POLYMER CHAIN, AND HAVING AN AVERAGE NUMBER OF CARBON ATOMS OF AT LEAST TWO IN THE SEGMENTS OF THE CHAIN SEPARATING THE AMIDE GROUPS. 